Quantum Entanglement

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Quantum Entanglement

Quantum Entanglement is a property of some quantum systems with more than one particle. When two different particles are created in such a way that the properties of one depend on the other’s, the system is said to be entangled.

The best way to understand Quantum Entanglement is through an example. Let’s imagine that, through some process, two electrons are created. We know the total spin of the system should be 0, because we’ve measured it before. However, we do not know the spin of each of the individual electrons. In fact, before we perform a measurement, both electrons will have a state which will be a combination of both spin up and spin down.

The crucial property here is that, since the system’s spin has to be 0, this means that, if one electron’s spin is up, the other has to be down and vice-versa. However, before we perform a measurement, both electrons have a state which is a combination of both.

We can now separate these two electrons as much as we want: their properties will still depend on each other’s. If we now perform a measurement on one of the electrons and obtain the value “spin up”, the other electron’s spin will be automatically determined to be “down”, instantaneously. This seems like action at a distance -and in a way it is- but, since there is no way to communicate the information to the other observer faster than light, there is no violation of causality.

Quantum entanglement can be easily interpreted within the many-worlds view: when we make a measurement on one of the electrons nothing really changes for the two electrons: it changes for us, because we become entangled with one of the states of the system (be it up-down or down-up).

See the Wikipedia page on Quantum Entanglement.

Leonard Susskind’s lecture at Stanford University.

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